1. Field of the Invention
The present invention relates to a method for manufacturing an electrode in such a manner that scraps generated in a process for manufacturing electrodes and an electrode integrated in a spent cell are reused.
2. Prior Art
Hitherto, lead storage batteries and nickel-cadmium batteries have been known which are secondary batteries arranged to repeatedly be recharged/discharged. Moreover, a nickel-hydrogen storage battery, a lithium ion secondary battery and the like have been developed in about 1991, and then brought to the marketplace. Since the lithium secondary battery exhibits a high energy density, even though the lithium secondary battery is a costly battery, the lithium secondary battery is employed as a power source for a portable electronic apparatus, such as a camcorder, a portable telephone or a notebook personal computer. Moreover, the lithium secondary battery is expected to serve as a large-size secondary battery in the form of a power-storage-type battery for an automobile.
At first, the lithium secondary battery includes lithium metal serving as a negative-electrode active material. Since lithium has the most basic potential among all of metal materials and a low specific gravity of 0.534, use of lithium to serve as the negative-electrode active material is theoretically expected to obtain a great energy density.
If the lithium negative electrode is repeatedly subjected to a recharging and discharging cycle, it is changed into a significantly reactive granular shape. Thus, a satisfactorily long cycle life cannot be obtained. What is worse, lithium can grow into dendrite crystal which can reach the positive electrode. Thus, there arises a problem in that internal short is induced.
Therefore, investigations have been performed to use lithium in a stable form of lithium ions by occluding lithium into a host material. As a result, a lithium-ion secondary battery containing a carbon material serving as the host material (the negative-electrode active material) has been developed.
Lithium ions cannot easily be occluded in the carbon material in an industrial viewpoint. Therefore, lithium-ion batteries which have been put into practical use are arranged in such a manner that the carbon material is introduced into the battery in a state in which lithium ions are not occluded. After the battery has been assembled, the battery is electrically charged so that lithium ions are occluded in the carbon material.
Therefore, the positive-electrode active material must originally contain lithium ions. For example, lithium-containing composite oxides, such as cobalt acid lithium, nickel acid lithium and spinel manganese acid lithium, have been employed.
To constitute an electrode by using the foregoing carbon material and the lithium-containing composite oxide, the foregoing materials in the form of granular shape are mixed with a binder and organic solvent so that a mixed coating material is prepared. The prepared mixed coating material is applied to the surface of a collector (a conductive secondary battery), and then compression-molded.
The lithium-ion secondary battery including the above-mentioned electrode is free from fining of the negative electrode and growth of lithium into dendrite crystal. Thus, a high energy density can be realized, the safety can be improved and a long cycle life can be realized.
However, the lithium-ion secondary battery put into practical use has a multiplicity of problems which must be solved.
Although the energy density can be raised because of proper use of an active material, the realized energy density has not reached a theoretical energy density.
A technique for reusing scraps generated in the process for manufacturing the electrode and the electrode integrated in a spent cell is an important factor for the purpose of preventing environmental destruction. In particular, resources of cobalt which is the raw material of the cobalt acid lithium are insufficient and thus cobalt is a costly material. Therefore, cobalt must be reused.
Hitherto, cobalt acid lithium and nickel acid lithium have been subjected to various processes including a process for roasting the batteries and crushing, magnetic separation and screening processes. Thus, fractional collection of nickel or cobalt has been performed. As an alternative to this, electrodes have been heated and oxidized so that cobalt acid lithium and nickel acid lithium are collected.
However, the fractional collection of nickel or cobalt, which is a costly process, cannot be employed in the industrial viewpoint.
Although electrodes can be heated and oxidized with a low cost, the characteristics of the active material are changed during the hot oxidization process. Therefore, a secondary battery containing the recovered active material suffers from deterioration in the characteristics as compared with a new battery.